1. Field of the Invention
The present invention relates to high speed digital integrated circuits, and in particular, to low voltage differential swing (LVDS) signal drivers for uses in and with high speed digital integrated circuits.
2. Description of the Related Art
With the tremendous growth of the Internet, data transfers, in terms of both volume and speed, are increasing dramatically in all areas of communications. For example, data streams for digitized video signals, high definition television (HDTV) and color graphics data require increasing amounts of bandwidth. As a result, increasingly higher speed interconnects between integrated circuits (chips), functional boards and systems become increasingly critical. While virtually all such data is digital in form, it is a high speed analog circuit technique that has become increasingly prevalent in meeting such data transfer needs. This circuitry, i.e., LVDS, provides for multigigabit data transfers on copper interconnects and high speed transmission lines, including fiber optic applications. These LVDS circuits have proven speed, low power, noise control and cost advantages important in point-to-point applications for telecommunications, data communications and video displays.
However, while LVDS circuits continue to provide significant advantages in applications requiring high data transfer rates, such circuits are not immune from three major parameters that influence the operation of virtually any circuit or system: circuit fabrication (or manufacture) process variations (xe2x80x9cPxe2x80x9d); power supply voltage variations (xe2x80x9cVxe2x80x9d); and operating temperature variations (xe2x80x9cTxe2x80x9d); often referred to collectively as PVT.
With respect to fabrication process variations, it is well known that notwithstanding the stringent quality control measures typically used to fabricate integrated circuits, fabrication processes nonetheless suffer some variations among the various processing parameters.
With respect to power supply variations, it is well known that notwithstanding the use of various filters or shielding techniques, noise and especially low frequency noise can be present or induced in the power supply line (e.g., switching noise, electromagnetic interference, etc.). Power supply noise can cause jitter on the rising and falling edges of the signal being processed, as well as frequency skew within the output signal.
With respect to operating temperature variations, such variations will virtually never be avoidable, as operating temperatures can vary due to a number of causes, including variations in data transfer rates, ambient temperature, variations in power supply voltage, among others. As operating temperatures vary, so can the amplitude, phase and frequency of some of the signals being processed.
Further, conventional LVDS circuits are sensitive to circuit load conditions. Variations in the load impedance will induce variations in the output differential voltage or output offset voltage or both.
A low voltage differential swing (LVDS) signal driver having a substantially constant output differential voltage (Vod) and a substantially constant output offset voltage (Vos) irrespective of variations in circuit fabrication processes, power supply voltages and operating temperatures (PVT), as well as circuit load conditions. A driver replica circuit which replicates a portion of the actual LVDS driver circuit conducts a driver replica operating current that is a scaled replica of the LVDS driver operating current. Operating voltages within the LVDS driver and driver replica circuits are monitored and controlled by bias voltages provided by the driver replica circuit. The desired scaling factor for the operating currents is ensured by appropriate scaling of the sizes of the circuit devices within the LVDS driver and driver replica circuits.
In accordance with one embodiment of the presently claimed invention, a low voltage differential swing (LVDS) signal driver includes differential signal driver circuitry and signal replication circuitry. The differential signal driver circuitry receives upper and lower biasing signals and in response thereto conducts a driver operating current, provides a driver monitor signal and receives and converts a differential input signal to a LVDS signal. The signal replication circuitry, coupled to the differential signal driver circuitry, receives upper and lower reference signals and the driver monitor signal and in response thereto provides the upper and lower biasing signals and conducts a replica operating current which is maintained as a predetermined replica of the driver operating current.
In accordance with another embodiment of the presently claimed invention, a low voltage differential swing (LVDS) signal driver includes differential signal driver means and signal replicator means. The differential signal driver means is for receiving upper and lower biasing signals and in response thereto conducting a driver operating current, providing a driver monitor signal and converting a differential input signal to a LVDS signal. The signal replicator means is for receiving upper and lower reference signals and the driver monitor signal and in response thereto providing the upper and lower biasing signals and conducting a replica operating current which is maintained as a predetermined replica of the driver operating current.
In accordance with still another embodiment of the presently claimed invention, a method for generating a low voltage differential swing (LVDS) signal driver includes: receiving upper and lower biasing signals and in response thereto conducting a driver operating current, providing a driver monitor signal and converting a differential input signal to a LVDS signal; receiving upper and lower reference signals and the driver monitor signal and in response thereto providing the upper and lower biasing signals and conducting a replica operating current; and maintaining the replica operating current as a predetermined replica of the driver operating current.